Dimensioning and Grid Integration of Mega Battery Energy Storage System for System Load Leveling
1 Dimensioning and Grid Integration of Mega Battery Energy Storage System for System Load Leveling Gauthier Dupont, Member, IEEE, and Philipp Baltus thermal units. The burden on the environment and the Abstract--In countries without hydropower potential, Peak economy is heavy.load and spinning reserve have to be entirely covered by thermal Until recently UAE strategy was to invest in combinedgeneration. The corresponding environmental and economic cycle units, which are there the cheapest thermal solution. Butburden is increasing on and on. This paper discusses how using with the outstanding economy growth in the region the powermega Battery Storage Energy System (BESS) for system loadleveling can mitigate the problem, and to which extent. sector can barely follow the move and new capacityLimitations on peak shaving of load profiles with flat and long installation reaches unprecedented highs .peak are identified and quantified. It proposes a simple empiric It clearly appears to the UAE authorities that alternativemethod for dimensioning the capacity of mega BESS connected to solutions have to be found. A quick overview of the situationprimary substations (S/S) with loads. The method is based on allows drawing up that one unique technology will not be abledetailed load following operation calculations performed for theUnited Arab Emirates. The concepts of S/S peak inversion and alone to mitigate the problem. Recently several projects werereduced S/S firm capacity are introduced. Finally, simplified initiated, and some of them are already under implementation.scheme for grid integration of mega BESS to S/S without loads is To name the majors: demand side management (DSM),proposed. distributed generation, smart grid, renewable energy, distributed BESS and centralized BESS. BESS solution was Index Terms— Batteries, energy storage, load leveling. addressed quite recently and benefits from new battery and power conversion system (PCS) technologies that are now I. INTRODUCTION mature and available on the market .N ew technologies in Battery Energy Storage Systems (BESS) could in a near future deny the cliché that energycannot be stored in large amount. Engineers are indeed BESS are here classified into two categories: distributed systems and centralized systems. Distributed BESS are big systems (≥ 1 MW) aiming toconfronted with this annoying constraint since the birth of the improve the system and reduce the costs at medium voltagepower sector. An adverse consequence is that networks have to (MV) system level (local load leveling, renewable energybe dimensioned for a safe load supply during peak load hours, storage, power quality, back-up supply during gridwhich occur a few hours a day for a few months only. For the disconnection, reliability improvement, upgrades deferral, etc.)rest of the year, the networks are well oversized. [2-4]. Charge and discharge strategy complies with local Load leveling can mitigate the problem. Cheap excess base constraints and objectives. Almost all existing systems fallload capacity is stored during low load conditions and released under this category.during peak hours, reducing expensive power generation. For Centralized BESS on the contrary are much bigger systemsdecades, pumped storage hydroelectricity was the solely (≥ 20 MW), i.e. mega systems, aiming to support the HV grid,practical solution. Unfortunately, appropriate sites are limited by reducing the system peak load thanks to large-scale loadin number. Some countries even do not have any site. This is leveling and/or by participating to spinning reserve. The twoespecially the case for countries without hydroelectricity world largest centralized systems are found in Fairbanks,potential, like the United Arab Emirates (UAE). Alaska, USA (40 MW for 7 minutes – only spinning reserve) In countries without hydroelectricity potential, covering the and in Japan for the Rokkasho 51 MW Wind Farm (34 MWpeak load demand is excessively expensive (install capacity for 7.2 hours).investment and operation costs), because it has to be entirely Both types can also offer additional functions like areaproduced by thermal units. Spinning reserve requirements regulation, long line stabilization, power factor correction,worsen the problem, because it has also to be produced by frequency regulation, voltage control, black start capability, carbon emissions reduction, etc. [4-8] This paper is focused on large-scale load leveling, which is G. Dupont is with the Department of Power Systems MVV decon GmbH,Bad Homburg, Germany (e-mail: email@example.com). the main objective of BESS installation in UAE. Other P. Baltus is with the Branch Middle East of MVV decon GmbH, Abu functions are also considered as additional benefits for theDhabi, United Arab Emirates (e-mail: firstname.lastname@example.org).
2country but are not discussed here. They includes spinning completely flat. Peak shaving equals the BESS capacity whenreserve participation (except when the system is discharging at the peak is sharp and lasts only a few hours (Fig. 1). On thefull capacity), and in a less extent frequency regulation and contrary, if the peak load is flat and lasts for many hours (Fig.voltage control. 2), the BESS capacity is dimensioned for the charge and the In Section II the use of mega BESS for full system load peak shaving is smaller. Profile 1 is taken from an Africanleveling is presented. Limitations on peak shaving for flat and country. Profile 2 is taken from an Emirate.long peak are identified and quantified in Section III. SectionIV proposes a method for dimensioning the capacity of mega MW 5000BESS connected to primary substations (S/S) with loads.Section V and VI introduce respectively the concepts of S/S 4000peak inversion and reduced S/S firm capacity. Section VIIdescribes the grid integration of mega BESS to S/S without 3000 System Load without BESSloads. Finally Section VIII qualitatively presents the benefits System Load modified by BESSof load leveling. 2000 BESS Charge/Discharge II. SYSTEM LOAD LEVELING 1000 Like any large-scale energy storage system, a mega BESS 0allows storing cheap base-load energy produced typically atnight and releasing it during peak hours, every day. The -1000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24resulting modified load profile is flattened, the load factor hourincreases and the peak load is shaved. TABLE I Fig. 2. System load leveling – Profile 2 LOAD PROFILE CHARACTERISTICS Load following operation (LFO) simulation  are Profile Name Profile 1 Profile 2 performed with a charging/discharging efficiency factor of Peak Duration < 6h > 8h 73%. About 8 and 5% of the load demand have to be stored in Peak Load MW 5.000 5.000 the BESS respectively for Profiles 1 and 2. Corresponding Bottom Load MW 3.000 3.920 peak shaving comes to 24 and 9% (Table II). These figures Load Factor p.u. 0,75 0,90 show the interest of BESS, especially for load profiles with a Demand MWh 89.900 107.927 sharp and short peak. BESS installed capacity required for completely even out TABLE IIthe system load depends on the load profile (Table I), the LOAD LEVELING RESULTSefficiency and charging/discharging characteristics of the Profile Name Profile 1 Profile 2battery system. BESS Cap. MW 1180 630 Bmax % of peak 23.6 12.6 Max. BESS MW 820 630 MW 5000 Charge % of peak 16.4 12.6 4000 Max. BESS MW 1180 450 Discharge % of peak 23.6 9.0 3000 System Load without BESS % of BESS cap. 100 72 System Load modified by BESS 2000 BESS Charge/Discharge BESS load MWh 6750 4791 (inc. loasses) % of load 7.5 4.4 1000 Load Demand MWh 4970 3518 Shaving % of load 5.5 3.3 0 Charge h 15 12 -1000 Discharge H 9 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Efficiency % 73.6 73.4 hour III. PEAK SHAVINGFig. 1. System load leveling – Profile 1 When the BESS capacity is much smaller than the load, one MW of installed capacity exactly results in one MW of peak Given a daily system load profile, the maximum BESS shaving. For load profiles with a sharp peak, this linearcapacity Bmax is reached when the modified load profile is relation is maintained until reaching full load leveling. For
3load profiles with a flat peak lasting for many hours, the 54% of Bmax. It corresponds to 6.8% of the peak load, i.e.relation is not linear beyond a certain BESS capacity value about 70 MW of BESS per GW of peak load. Such a figureBlim. corresponds to mega BESS. 500 IV. BESS CAPACITY DIMENSIONING IN S/S WITH LOADS Mega BESS considered in UAE comprises several BESS 400 blocks. Each block (Fig. 5) is composed by a large number of battery cells grouped in series into parallel strings. Several Peak shaving (MW) strings are connected in parallel to a common DC bus 300 connected to a PCS. At their turn, Several PCSs are connected in parallel to a common AC bus. Finally a step-up transformer 200 connects the AC bus to the grid. The HV side of the transformer is the grid terminal of the corresponding BESS block. 100 The number of strings and of PCS put in parallel, as well as the power ratings and the nominal voltages of major BESS 0 components – batteries, PCS, transformers – depends on the 0 100 200 300 400 500 600 chosen battery and PCS technology (maximum string voltage, BESS capacity (MW) maximum number of parallel strings, PCS AC and DCFig. 3. Peak shaving (MW) vs. BESS capacity – Profile 2 voltages), the objective of the project (stored energy for load leveling, peak shaving, grid AC voltage) and available Beyond this point, the energy that can be charged is not equipments on the market.sufficient to balance the losses and the energy to be dischargedfor load leveling. It is due to the long discharging period andthe small power difference between low and peak load.Discharged energy has then to be reduced. One can limit thedischarge period or reduces the discharge output power on thewhole discharge period. The second solution is chosen becauseit results in a uniform peak shaving. LFO simulations forprofile 2 show that the reduction equals 72% for the completeload leveling (Fig. 3). 100,0 Peak shaving Psh (% of BESS capacity) 95,0 90,0 85,0 80,0 75,0 Fig. 5. Generic single-line diagram of one BESS block 70,0 0,0 10,0 20,0 30,0 40,0 50,0 60,0 70,0 80,0 90,0 100,0 BESS capacity B (% of B max ) Each block comprises a control system, which integrates all specific control sub-systems (battery modules, PCS,Fig. 4. Peak shaving (%) vs. BESS capacity – Profile 2 transformer, AC and DC circuit breakers, etc.) A central control system pilots the control systems of the blocks LFO simulations performed for various BESS capacity composing the mega BESS. On a communication and controllevels show that the relation between peak shaving Psh (in point of view the BESS then acts like a single unit.percent of BESS capacity) and the BESS capacity B (in MW A mega BESS is thus seen by the grid as a singleor in percent of its maximum Bmax) is linear (Fig. 4): controllable power unit but with several grid terminals, that can be connected to the same S/S or geographically Psh = a ⋅ B + b for Blim ≤ B ≤ Bmax distributed. Network integration study has then to fill the gap The relation is independent of the absolute peak load value. between the BESS terminals and the grid, by choosingFor Profile 2, a = -0.61 and b = 133. At Psh = 100%, B = Blim = appropriate locations (space and spare feeders availability, S/S
4load and firm capacity, etc.) UAE are installed with four 220/33 kV 140 MW transformers An Emirate’s network comprises about fifty HV/MV offering a firm capacity of 380 MW. Given a typical value ofprimary S/S (HV: 220 or 132 kV – MV: 11 or 33 kV). αbottom = 0.6 and a conservative value of αLP = -5, maximumConnecting mega BESS blocks to a S/S is feasible by simply BESS capacity Bfirm is then about 140 MW.connecting with cables each block terminal to a MV feeder ofthe S/S. Such a grid connection is possible at the condition that V. SUBSTATION PEAK INVERSIONenough spare feeders are available or that substation Hourly simulations for a BESS with an installed capacityextensions are possible, and that enough space for the BESS is equals to Bfirm and connected to a S/S with loads are presentedavailable close the S/S. Because of the large footprint of a in Fig. 6 and 7. They present two different discharge strategies.BESS, the last condition could be difficult to fulfill, especially The S/S load profile with BESS is characterized by a peakfor existing S/S in already developed areas. occurring during system low loads (at night) and a low load taking place during system peak load hours. The S/S peak and TABLE III αBOTTOM AND αLP CALCULATIONS RESULTS low loads are inverted. The S/S with BESS contributes to system load leveling. This behavior is called substation peak αbottom Rural areas 0.4 to 0.6 inversion. Industrial areas 0.7 to 0.8 400,0 Cities 0.6 to 0.7 αLP -20 to 20% 300,0 B firm 200,0 The maximum S/S power transfer from the HV grid to itsMV side equals the firm capacity of the HV/MV transformers MW 100,0Pfirm. In UAE, Pfirm is defined with N-1 contingency criteria 0,0and a load factor between 0.85 and 0.9 depending on the loads B firmtype. -100,0 Maximum BESS capacity in an S/S with loads Bfirm is -200,0limited during charging period by the difference between Pfirm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24and the bottom load Pbottom (Fig. 6). Detailed LFO calculations hour S/S Load without BESS S/S Firm Capacity (P firm)are performed for every S/S. Analysis of the results allows S/S Load with BESS BESS Power Outputobtaining the following simple empiric equation: B firm = Pfirm ⋅ (1 − α bottom ) ⋅ (1 + α LP ) Fig. 7. Substation peak inversion 2 – BESS capacity = Bfirmwhere αbottom = Pbottom / Pfirm and αLP is a correction factor that VI. REDUCED SUBSTATION FIRM CAPACITYtakes into account the BESS charging/discharging Consider a BESS connected to a S/S with loads with ancharacteristics and the load pattern. The factors αbottom and αLP installed capacity B greater than Bfirm. In order to comply withare calculated for all S/S. Table III presents typical values. N-1 contingency on an event on one of the HV/MV transformers, the firm capacity available for the loads of the 400,0 S/S has to be reduced (Fig. 8). B firm 300,0 400 200,0 350 B P red 300 MW 100,0 250 0,0 MW B firm 200 -100,0 150 -200,0 S/S Firm Capacity (P firm) 100 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 S/S Load without BESS hour 50 S/S Bottom Load (P bottom) S/S Load without BESS S/S Firm Capacity (P firm) 0 S/S Load with BESS BESS Power Output 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 hourFig. 6. Substation peak inversion 1 – BESS capacity = Bfirm Fig. 8. Reduced firm capacity Pred This empiric method allows easily estimating the maximumBESS capacity that can be installed in a S/S with loads, As for Bfirm, an empiric equation allows estimating thewithout requiring detailed LFO simulations. The biggest S/S in reduced firm capacity Pred:
5 B Pfirm − Pred = (1 + α LP ) HV Line α bottom 220 kV Given B = 300 MW, Pfirm = 380 MW and the typical valuesof αbottom and αLP given here above, then Pred falls down to 140 MVAaround 110 MW, that is a reduction of 270 MW. In this case,S/S peak inversion is much significant and therefore system 33 kVload leveling contribution is bigger (Fig. 9). 20 MW 20 MW 20 MW 20 MW 20 MW 20 MW 20 MW 20 MW 20 MW 20 MW 400,0 300,0 200,0 BESS 100,0 MW 0,0 Fig. 10. Example of a 200 MW BESS in a S/S without loads -100,0 -200,0 VIII. LOAD LEVELING BENEFITS -300,0 In UAE, base load is produced by combined cycle units, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 hour which produce the cheapest possible electricity in a country S/S Load without BESS S/S Firm Capacity (P firm) without hydropower potential. And peak load is generally S/S Load with BESS BESS Power covered by backup fuels (crude oil, gas oil, fuel oil), which are up to five to six times more expensive than natural gas.Fig. 9. Substation peak inversion – BESS capacity > Bfirm With such a fuel cost difference, load leveling can help to reduce system production costs, even with an efficiency factor VII. BESS CONNECTION TO S/S WITHOUT LOADS of 70 to 75%. By also taking into account the generation Maximum BESS capacity Bfirm or reduced firm capacity Pred deferral, including the corresponding operation andconstraints do not apply if the system is connected to a S/S maintenance costs reduction, the benefits are then sufficient towithout loads. In this case, the S/S firm capacity is fully justify the investment of mega BESS in UAE.available for the BESS. Practically a new S/S is build only for Thanks to lower CO2 emissions with combined cycle thanBESS grid integration. with backup fuel, about 0.35 Tons of CO2 can be saved for As explained above, an original characteristic of mega each produced MWh. Trading these emissions savings on aBESS is that it is a single controllable power unit but with market like the European Union Emission Trading Schemeseveral MV terminals. A main advantage of having several (EU ETS)  increases the BESS benefits by about 0.5 toterminals is the increased reliability compared to a single 1%.terminal. Thanks to an adapted design, N-1 event is limited tothe loss of a single block (typically several tens of MW), all IX. CONCLUSIONthe other ones staying on-line (hundreds of MW in total). It This paper provides examples of mega BESS dimensioningmakes a mega BESS extremely reliable. for full system load leveling. Limitations on peak shaving for Fig. 10 schematically displays an example of possible load profiles with flat and long peak is identified andconnection at 33 kV of a 200 MW BESS to a S/S without quantified. Empiric methods for maximizing the use of firmloads. The BESS is composed of ten blocks of 20 MW each. capacity of HV/MV substations with or without loads forThe corresponding 20 terminals are distributed among three BESS dimensioning are also proposed. In order to do so, thedouble bus bar sections. Each section is supplied by a 220/33 concepts of S/S peak inversion and reduced S/S firm capacitykV 140 MVA transformer. All equipments are standards ones are introduced.used in typical S/S in UAE. This scheme completely satisfiesN-1 reliability constraints (transformers, section bus bars and X. REFERENCESlines). The worst N-1 event is the loss of a 20 MW block, i.e.  “TRANSCO 2008, 5-Year Electricity Planning Statement (2009-10% of the installed capacity of the whole system. 2013),” Asset Management Directorate, Power Network Development The big advantage of this solution compared to the Department, TRANSCO, UAE, November 2008 - www.transco.ae.  “EPRI-DOE Handbook of Energy Storage for Transmission andconnection to S/S with loads is the freedom for choosing Distribution Applications,” EPRI Report # 1001834, December 2003 -BESS location. Such a system can be installed almost www.epri.com.anywhere along the HV network, while for S/S with loads,  F. A. Chacra, P. Bastard, G. Fleury and R. Clavreul, “Impact of Energy Storage Costs on Economical Performance in a Distributionenough space has to be available close to the S/S.
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